Chromium base alloy possessing highstrength at elevated temperatures



United States Patent CHROMIUM BASE ALLOY POSSESSING HIGH- STRENGTH AT ELEVATED TEMPERATURES Lewis R. Aronin, Lexington, and Arthur L. Geary, Ar-

lington, Mass., assignors, by mesne assignments, to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed June 30, 1960, Ser. No. 40,123 1 Claim. (Cl. 75-176) The present invention relates in general to chromium rich alloys and is specifically directed to alloys having superior strength at high temperatures combined with adequate hot workability.

Chromium, because of its high melting point and oxi dation resistance, ofiers an attractive basis for high temperature alloys. Strengthening achieved by selected alloying elements should result in an alloy having useful strength as a structural material at temperatures well in excess of 1000 C. (1830 F.). While other metals such as molybdenum, tantalum and tungsten have higher melting points than chromium, the latter is superior to these in oxidation resistance.

In the search for strong, high temperature chromium alloys, the most satisfactory alloy has been reported as Cr-Fe-Mo in the range 60 w/o chromium, 15 to 20 w/o iron and 15-25 w/o molybdenum (Parke and Bens, ASTM Symposium on Materials for Gas Turbines, 1946).

Sully and Brandes, The Properties of Cast Chromium Alloys at Elevated Temperatures, Journal of the Institute of Metals, 81, 573 (1953), also investigated the strengths of cast chromium base alloys. Results of compression and creep tests at 900 C. (1650 F.) indicated that a Cr-l0 w/o Fe-lO w/o W alloy containing either 5 or w/o Ta was the most resistant to deformation when stressed at 10,000 psi. Erratic results, however, were obtained in tensile stress-rupture tests, this behavior being attributed to defects in the cast structure which obscured the intrinsic creep resistance of the alloys.

Chromium base alloys containing 1 w/o Ti and up to 5 w/o W prepared by are melting mixture of hydrogentreated electrolytic chromium and high purity titanium and tungsten powders have been successfully forged, swaged and rolled (Henderson, Quass and Wain, The Fabrication of Chromium and Some Dilute Chromium- Base Alloys, Journal of the Institute of Metals, 83, 126 (1954). Other workers report that Cr-5 w/o Mo and Cr-lO w/o W alloys fabricated by extrusion of arc-cast ingots, possess attractive creep and stress-rupture properties at 950 C. (1740 F.) (Wilms and Rea, Preliminary Investigations on the Properties of Chromium and Chromium Alloys at Elevated Temperatures, Journal of the Institute of Metals, 87, 77 (1958).

It is the object of this invention to provide chromium alloys which have superior strength at high temperatures, are hot workable and extrudable.

These objects are obtained by alloying the chromium with small quantities of columbium, tantalum, and/or molybdenum. The alloys are distinguished from the prior art in that high-temperature strength is achieved with small alloying additions. Only two of the alloys of this invention have alloy contents as high as 7 w/o, while the majority have alloy contents substantially less than 5 W/o. The alloys can be hot worked and are extrudable into a wrought product.

FABRICATION OF ALLOYS The alloys of the subject invention are fabricated by powder metallurgy techniques. Blended powders, compacted at 50,000 p.s.i. into 1-inch diameter by 2-inch high slugs, are sintered for 3 /2 hours in purified argon at 2685-2730 F. (1475-1500" C.). The sintered slugs are enclosed in type 304 stainless steel containers and evacuated to 0.01 micron while heating at temperatures up to 800 F. (430 C.). After heating one hour at 2400 F. (1315 C.), the billets are extruded from a 2.800" liner of a 1000-ton press through 0.800 inch dies. The alloy core so produced is approximately .25 inch in diameter.

Alternatively, the slugs may be sintered in hydrogen. Slugs sintered in hydrogen are evacuated one hour at 1830 F. (1000 C.) to reduce residual hydrogen. In these alloys high purity chromium is used.

TESTING THE ALLOYS In order to test the alloys at a temperature of 2300 F. (1260 C.) a conventional stress-rupture test machine was modified to permit use of an inert (argon) atmosphere. Specimens are placed in grips and inserted in a tubular furnace which is comprised of a Kanthal winding on a mullite tube. The latter is sealed, flushed with argon, after which the furnace is turned on. Within about 5 hours the specimen reaches the test temperature, 2300 F. (1260 C.). After soaking for half an hour at temperature, a load is applied to the specimen and the rupture life is measured to the nearest 0.1 hr. by an electric timer.

The properties of unalloyed chromium and 23 chromium-base alloys have been investigated in stress-rupture tests at 2300 F. (1260 C.). Results for unalloyed chromium and for chromium alloys are set forth below in Table I and Table 11 respectively:

Table I.--Results of Stress-Rupture T ests at 2300 F. (1260 C.) Unalloyed Chromium- COMPACTED POWDER-S EXTRUDED TO 0.18-INCH DIAMETER Stress Elongation Reduction Rupture Atmosphere (p.s.l.) (percent) in area life (hr (percent) SINTERED POWDERS EXTRUDED TO 0.18-INCH DIAMETER Argon 1, 720 81. 2 68. 7 5. 8 D 1, 220 55. 3 18. 2

COMPACTED POWDERS EXTRUDED TO 0.25-INCH DIAMETER COMPACTED POWDERS EXTRUDED ETER; HEAT TREATED 2 HOURS 2,460 F. (1,350 C.)

TO 0.25-INOH DIAM- IN HYDROGEN AT are valid, and the data summarized in Table III is deemed to be correct within reasonable-limits.

Table III.-Life at 1000 p.s.i. and Stress to Rupture in 100 Hours at 2300 F. Unalloyed Chromium and Chromium-Base Alloys Life at 100 hour Alloy composition (alo) 1,000 p.s.i. rupture (hr.) stress (p.s.i.)

68 830 64 800 64 800 (54) (770) 50 840 48 551 (Jr-1.0 Cb-1.0 Ta I 48 770 (Jr-1.0 Pa-0.1 Ola-0.1 M 35 550 Cr-0.6 Zr-1.0 Ch..." 33 590 Cr-0.6 Zr-1.0 M0 33 590 Cr-O 6 Zr-LO Ta.- 33 590 Cr-l 1 'Ii-1.0 Cb 23 510 r 2 0 Ti 23 510 Cr-l 1 Ti-1-0 Ta 23 610 Cr-LO T' 12 320 Unalloyed chromium:

Compacted, 0.18-inch core (17) (600) compacted, OAS-inch core and tested in helium 17 600 Sintered, 0.18-inch core. (33) (710) compacted, 0.25-inch core.--" 450 Compacted,0.25-tnch core and h ttr ted. 10 450 Sintered, 0.25-inch core (71) (910) I Doubtiul values enclosed in parentheses.

"From Table III it Will be seen that appropriate additions of Cb, Ta and Mo have resulted in rupture lives 3 is the alloy Cr-l a/o Cb0.1 a/o Mo which appears to conditions.

be the strongest alloy tested except perhaps the Cr-2 a/o Ta-0.1 a/o Mo as to which the data is scattered so that its life at 1000 p.s.i. cannot be precisely stated.

A number of other alloys successfully meet the desired These include (in atomic percent) Cr1.0 Cb; Cr-1.0 Cb-0.1 Ta;'Cr-2.0 "Fa-0.1 Cb-O.l Mo; Cr1.0 Ta-0.l Mo; and Cr-1.0 Cb-LO Mo.

From the foregoing it is seen that new and useful alloys of chromium have been produced by the addition of small amounts of selected alloying elements whereby the strength of chromium has been substantially increased at stresses of about 1000 p.s.i. and temperatures of about 2300 F.

We claim:

An alloy of chromium characterized by high strength at temperatures of about 2300 F. that consists of chromium-1 atomic percent columbium, and 0.1 atomic percent molybdenum.

References Cited in the file of this patent Kubaschewski et al.: Journal Inst. Metal., vol 75, pp. 407-413 and 419, 1949.

Sully: Chromium, Butterworths Scientific Publication, London, 1954, p. 239.

Abrahamson et al.: Brittle to Ductile Transition Temperature of Binary Chromium-Base Alloys, A.S.M. Transaction, vol. 50, 1958, pp. 709 and 7ll.

Hansen: Constitution of Binary Alloys, McGraw-Hill Book Co., Inc., New York, 1958, p. 541. 

